The spectral output of a MOPA system used for UV optical lithography includes a main emission “line” of laser radiation having a spectral width less than 1.0 picometers (pm) centered about a peak intensity wavelength and amplified spontaneous emission (ASE), having a spectral width between about 300 pm and 500 pm distributed about the peak intensity wavelength of the emission-line. The ASE has orders of magnitude lower intensity than the intensity of radiation in the main emission-line. In operations such as microlithography, suppression of total (wavelength integrated) ASE power below a few times 0.01% (1×10−4) the total power of the main emission-line energy is usually required. Because of this, measurement of ASE is important in determining whether or not the ASE is adequately suppressed.
Prior art methods of measuring ASE use a spectrometer to measure light intensity as a function of wavelength. One problem with using a spectrometer for ASE measurement is that light intensity is integrated over a wavelength band of about 10.0 pm around the peak emission wavelength (“in band” radiation), and integrated over the wavelength range of ASE outside of this 10 pm band. Since the integration range for ASE is usually at least 50 times the “in-band” integration range, for measuring integrated ASE of 1×10−4 or less, the spectrometer must have a multi-channel detector having an amplitude resolution on the order of greater than 1×106 in order to provide adequate signal-to-noise ratio. Such multi-channel detectors are either not readily available, or prohibitively expensive.
Another problem with using a spectrometer for ASE measurement is that spectrometers have limited contrast when used for this purpose. This is because high intensity radiation of the main emission-line scatters randomly on various optical components inside the spectrometer, thus creating a broad background on the detector. This background light can be indistinguishable from the input ASE light. This problem is very difficult to overcome. There is a need for an ASE measurement scheme that does not require such an extremely sensitive detector and does not have a problem discriminating against background scattered light.